Although the National Institute of Occupational Safety and Health (NIOSH) has identified a clear epidemiological link between repetitive arm motion and shoulder disorders in the workplace, there are few scientific data available regarding the biomechanics of this connection. This is surprising considering that occupational shoulder disorders have a direct medical cost of 4 billion dollars annually in the United States. There is evidence that repetitive motion is associated with muscle fatigue and abnormal shoulder motion, which in turn may lead to damage of the rotator cuff musculature. Based on this observation, the overall objective of this research proposal is to develop a simple index of muscle fatigue that is associated with repetitive arm motion and altered scapular kinematics. More specifically, the first specific aim is to determine if repetitive arm motion will lead to altered shoulder kinematics. It is hypothesized that fatigue of the scapular muscles during a repetitive arm motion will result in a decrease in scapular motion. The second specific aim is to identify a fatigue index to explain altered kinematics with repetitive motions. A fatigue index based on a muscle's EMG frequency components has been shown to be a good predictor of muscle impairment for low back muscles. It is hypothesized that a similar index for scapular muscles will be associated with alterations in scapular kinematics after repetitive motion. These hypotheses will be tested by having healthy volunteers with no shoulder pathologies perform a simulated work activity until they are fatigued. Based on the findings of NIOSH, this will be a high repetition and low load activity. Kinematics will be measured with a magnetic tracking device that has been found to be both reliable and accurate for measuring scapular motion. Fatigue susceptibility will be identified with a previously established muscle fatigue index. This proposal represents the first step towards developing a screening tool for assessing an individual's ability to resist the potentially harmful consequences of repetitive motion. Ultimately, this information will be used to develop both strategies for modifying work tasks, as well as conditioning programs for workers performing high risk activities.